Corrosion inhibitors in absorption refrigeration system



Jan. 1, 1952 N. E. WIDELL 2,580,933

CORROSION INHIBITORS IN ABSORPTION REFRIGERATION SYSTEM Filed Feb. 8.1946 IN V EN TOR.

Patented Jan, 1, 1952 CORROSION INHIBITORS IN ABSORPTION REFRIGERATIONSYSTEM Nils Erik Widell, Stockholm, Sweden, assignor to AktiebolagetEiektrolux, Stockholm, Sweden, a corporation of Sweden ApplicationFebruary 8, 1946, Serial No. 646,394 In Sweden January 22, 1945 15Claims. (Cl. 62-179) This invention relates to absorption refrigeratingapparatus and more particularly to such apparatus of the inert gas typewherein the absorption solution contains a corrosion protectivesubstance. The main object of the invention is to increase the lifetimeof the apparatus and to prolong the durability of the corrosionprotection. The invention is of particular importance in suchrefrigerating apparatus wherein absorption solution from its normalcirculating system through the boiler and absorber of the apparatus isbrought into contact with working medium having a higher refrigerantconcentration than that of the absorption solution.

The invention will be hereinafter more fully described with reference tothe accompanying drawing showing diagrammatically an embodiment of thearrangement according to same and in this connection furthercharacteristic features of the invention will be set forth.

The figure shows diagrammatically an absorption refrigerating apparatus,wherein water is used as an absorbent and ammonia as refrigerant.Referring to the figure reference numeral l denotes a tube acting as aflue, to which heat is continuously supplied from a heat source, notshown, for example an electric cartridge, 9. gas or petroleum burner.The boiler of the apparatus is designated by I I and its liquidcirculating pump by I2. The boiler pipe II is extended upwardly and atthe top connected to the air cooled condenser I3 of the apparatus.Further numeral I4 denotes a so-called pressure equalizing vessel of akind known per se which through a drainage conduit I5 communicates withthe evaporator of the apparatus designated by I6. In said conduit I5 thecondenser pipe I3 opens, from which liquid refrigerant thus flows bygravity to the evaporator through the conduit I5. Further numeral Ildesignates the gas heat exchanger of the apparatus and I8 its air-cooledabsorber. Numeral I9 indicates a vent conduit through which the pressureequalizing vessel I4 freely communicates with the gas circulating systemof the apparatus. Said system is formed in a way known per se as is theliquid circulating system of the apparatus. The enriched solution in theabsorber vessel 20 of the apparatus thus streams through its liquid heatexchanger 2| and the main pump l2 to the boiler I I and thence itreturns through the heat exchanger 2i and a conduit 22 to the absorberI8.

In accordance with usual practice the apparatus is formed of iron pipesjoined together by gas welding. Before filling the apparatus it is iiievacuated of all air, and as ammonia by itself has no influence uponiron in absence of oxygen, no corrosion should be feared in the internaliron walls. However, these walls formed of ferrous metal willnevertheless be affected by the ammonia solution by the forming ofcomplicated iron compounds, by which the walls after a relatively shorttime would be loosened up-and lose their strength against the highpressure (about 20 atm.) prevailing in the apparatus. In order to avoidsuch corrosion an alkali salt of sodium chromate (NazCrOr) haspreviously been dissolved in the absorption solution. The chromaticconcentration required in the absorption solution for counteracting thecorrosion is extremely small and may be counted in fractions of percent. However, the chromate is slowly consumed and therefore it haspreviously been necessary to utilize a concentration of chromate in thesolution many times higher for the purpose of creating a chromaticreserve, covering several years consumption. Salts dissolved in theabsorption solution will, however, influence the pumping in thethermosiphon pump I2 in a unfavourable direction and therefore until nowit has been necessary to limit the concentration of chromate in thesolution to a concentration corresponding to about 2% of 0103.

In different ways, however, it has been attempted toform an additionalreserve of alkali chromate by enclosing in the absorber vessel a storageof solid sodium chromate in a container with a limited communicationwith the surrounding solution, whereby a gradual dissolving of the solidchromate has been attained. It has not been possible to make thedissolving so uniform that the concentration of the solution withsecurity can be kept below the values wanted. The velocity of dissolvinghas also appeared to be dependent upon a plurality of uncontrollablefactors, among other things the operating temperature which veryessentially varies with the climatic concentration under which theapparatus is operated. According to this invention a reserve of chromateof an arbitrary greatness may be arranged in the apparatus. Theinvention is substantially characterized thereby that the absorptionsolution of the apparatus is saturated with a salt of the chromaticacid, having a solubility in said solution not exceeding 2%. Further thesolubility of the present chromate may be as much as possibleindependent of the temperature, though certain variations with thetemperature may be permitted. A particularly suitable chromate isstrontium chromate (SrCr04). The solu- 3 bility of strontium chromate inthe absorption solution is so small that the saturated solution onlycontains 0.01 to 0.2% of CrOa.

Apparently this small quantity of chromate which originally, whenfilling the apparatus, is dissolved in the absorption solution will becomparatively rapidly consumed. It is therefore generally necessary toenclose a reserve of solid strontium chromate in the apparatus.According to the invention therefore a container 50 is arranged in theabsorber vessel 20 or possibly in another vessel interconnected with theliquid circulating system of the apparatus. The reservoir 50 maysuitably include a short piece of pipe, both openings of which arecovered with a fine-meshed iron net and in which a pressed tablet ofstrontium chromate is inserted. The container may suitably be attachedto the wall of the vessel by a strip or similar member which is attachedto the wall of the vessel and to the wall of the container 5|! bywelding. The support is denoted by El. When the liquid contents of theabsorber vessel is continuously renewed, the absorption solution willautomatically become saturated gradually as its contents of chromatedecreases during its passage through the absorber vessel 20. Thepressing of the solid strontium chromate into a tablet possesses theadvantage that no solid particles by mechanical influence of thestreaming solution will follow the liquid through the narrow passages ofthe apparatus. In order to improve the strength of the tablet smallamounts of sodium hydroxide may be mixed with the chromate when pressingsaid tablet.

As a ready mentioned, the invention is of particular impo tance in suchabsorption refrigerating apparatus in which the absorption solution isnow and then brought into contact with liquid refrigerant, theconcentration of which thus attains high values. This is the case in theabsorpto the main pump l2 the auxiliary pump 23 is d not immediatelyheat conductively connected with the flue ill but instead arranged inheat conductive relation with a hermetically closed heat transfer systemwhich is welded to the flue in along a welded seam 42. The systemcontains a heat transfer meduim with a high boiling point, possibly inaddition to an auxiliary medium with -a lower boiling point. Thetransfer medium is vaporized in the boiling part ll of the system andcondenses to liquid in a container where the condensate is accumulated.Some part of the vapours, however, give oil their heat contents to thepump pipe 23. After a certain period the system is boiled dry, and allthe condensate remains storaged in the container 30. This container isconnected by a flexible member 3| with the system, thus permitting it tobe raised for tilting the liquid contents. In this way a method ofcontrolling the heat transfer system is attained, thus causing the pump23 to operate whenever desired and after a certain period automaticallyto stop working. The solution raised by the auxiliary pump 23 streams inthe way already described into the evaporator where 4 entering from thecondenser 13, having an em.- monia concentration from 80 to 100%. Thesupply of condensate from the condenser I3 takes place continuously.This supply of absorption solution to the evaporator 16 has for itsobject, when required, to effect melting of ice formed thereon. It hasappeared that a solution of sodium chromate of 1 to 2 per cent cannot bemixed with ammonia having the concentration existing in the evaporatorwithout most of the salt bein precipitated into solidform. The saltprecipitated would collect in the evaporator and would very soon effecta blocking action and at the same time such precipitated salt for thefuture would be prevented from taking part of the corrosionprotection. Asolution of strontium chromate remains, even if it is saturated,approximately unaffected by an increase of the ammonia concentration andthe small amounts of chromate which possibly may be precipitated will bereplaced from the reserve provided in the absorber vessel.

Though the strontium chromate is very little so uble in the absorptionsolution, that is, in the ammonia water mixture, the concentration ofprotecting substance in this chemical compound is sufilclent to effect asatisfying corrosion protection. Other chromates, such as lead chromate,neutral or basic (PbCr04 or PbCr04, PbO) are practically unsoluble inthe ammonia water solution. Accordingly such compounds alone and bythemselves have no corrosion protective effect. According to theinvention, however, the active substance of the lead chromate-may befreed from the lead when in the absorption solution an activatingsubstance is dissolved, such as sodium hydroxide (NaOH) or any othersubstance which when disintegrated forms sodium hydroxide. According toanother phase of the invention, sodium hydroxide, for examp e 2%, isdissolved in the absorption so ution. This solu. t on will then effect adisintegration of the lead chromate. substantially according to thefollowing form:

The sodium chromate formed thereby will be di solved by the absorptionsolution and distributed into all parts of the circulation system of theabsorption solution. The lead compound NaHPbO-z is also soluble in theabsorption solution, and therefore will not eilect any disturbances.According to the small concentration of the sodium hydroxide in thesolution, the quantity of sodium chromate formed and dissolved thereinin accordance with the foregoing eouation will be small and will remainat a substantlally constant value as the reaction proceeds until alllead chromate has been dissolved.

In some cases the low concentration of the chromate in the absorptionsolution will be insufficient for effecting a satisfactory protectionagainst corrosion. According to a further phase of the invention,however, a further corrosion protective substance, such as the nitritesof alkali metals, for example, may be added to the absorption solutionand dissolved therein until a concentraticn of about 4% is attained.Such nitrites of the alkali metals may include ammonium, potassium orsodium nitrite. Preferably sodium nitrite (NaNOz) is used. Though thischemical compound by itself is known as being very unsta bleinvestigations have shown that the duration of the protective eilfect issuilicient for the above it, however, is mixed with refrigerantcondensate 7i} purpose and may be very much increased when a chromate,also a very diluted solution of chromates, is used in combination withthe nitrites. The concentration of chromates suillcient for effectingsuch an increase of the duration of diluted nitrite solutionscorresponds to about 0.02 to 0.2% CIOa. It is evident that nitritesolutions containing 0.02 to 4% NaNO-i and saturated with strontiumchromate or lead chromate is very suitable as absorption solutions inrefrigerating apparatus of the above described type, due to the factthat sodium nitrite when acting as a corrosion protective substance willbe disintegrated forming thereby gaseous products and sodium hydroxide.This sodium hydroxide will dissolve lead chromate as described above,and accordingly activate said compound which in itself is practicallyinsoluble and inactive. The sodium hydroxide and the sodium nitriterespectively accordingly act as activators for the lead chromate. Thesodium chromate dissolved thereby prevents the rapid disintegration ofthe nitrites.

It is evident that the invention is of greatest importance in connectionwith hermetically closed refrigerating apparatus of the type describedin connection with the figure. The lifetime of such apparatus issubstantially determined by the duration of the corrosion protection.According to the invention it is now possible to enclose in such aparatus a corrosion protective substance soluble in the absorbent anduseable in high concentration, and a second corrosion protectivesubstance acting as conservator for said first corrosion protectivesubstance and a reserve of solid corrosion protective substance, fromwhich an active part is dissolved by means of the disintegrationproducts of the first-mentioned corrosion protective substance. Due tothis fact the corrosion protection may be made effective for anextremely long duration.

I claim:

1. In the art of refrigeration employing an hermetically sealedabsorption refrigerating system formed of ferrous metal and having anabsorption solution circuit in which aqueous ammonia solution iscirculated through and between a place at which ammonia is expelled fromsolution and a place at which ammonia is absorbed into solution, theimprovement which comprises maintaining in the solution a supply ofsolid lead chromate, and providing in the solution an alkali nitrite.

2..The improvement as set forth in claim 1 in which sodium nitrite isprovided in an amount to obtain a concentration of from 0.02 to 4.0 percent.

3. In the art of refrigeration employing an hermetically sealedabsorption refrigerating system formed of ferrous metal and having anabsorption circuit in which an aqueous ammonia solution is circulatedthrough and between a place at which ammonia is expelled from solutionand a place at which ammonia is absorbed into solution, the improvementwhich comprises maintaining in the solution a supply of solid leadchromate, and making available in the solution an alkaline hydroxide.

4. The improvement as set forth in claim 3 in which about 2 per centsodium hydroxide is added to the solution.

5. In the art of refrigeration employing an hermetically sealedabsorption refrigerating system formed of ferrous metal and containingammonia, water and hydrogen and comprising an evaporator and a number,of parts interconnected to provide a circuit for circulation of aqueousammonia solution and in which such solution is supplied at will fromsaid circuit to the evaporator to effect defrosting of the latter whendesired, the improvement which comprises adding to the solution analkaline nitrite, and maintaining in said circuit a supply of a chromatesalt of lead which is in solid form.

6. The improvement as set forth in claim 5 in which sodium nitrite isadded to the solution.

7. In the art of refrigeration employing an absorption refrigerationsystem formed of ferrous metal and utilizing ammonia as refrigerant andwater as an absorbent, that improvement which comprises maintaining inthe aqueous ammonia solution in physical contact therewith a supply of achromate salt of lead soluble in the solution only with difllculty andhaving bound therein a chromate radical inactive due to the insolubilityof said lead salt, and adding to the solution another substance whichupon dissolution or disintegration will provide an alkaline hydroxide toeffect liberation of said chromate radical by chemical action.

8. In the art of refrigeration employing an absorption refrigerationsystem formed of ferrous metal and utilizing ammonia as refrigerant andwater as an absorbent, that improvement which comprises maintaining inthe aqueous ammonia solution in physical contact therewith a supply of achromate salt of lead soluble in the liquid only with difliculty andhaving bound therein an inactive chromate radical capable of forming a,readily soluble corrosion inhibiting chromate salt upon liberation fromsaid lead salt, and providing in the solution an alkali hydroxide whichwill eifect liberation of said inactive chromate radical from said leadsalt and promote formation of said corrosion inhibiting chromate salt.

9. In the art of refrigeration employing an absorption refrigerationsystem formed of ferrous metal and utilizing ammonia as refrigerant andwater as an absorbent, that improvement which comprises maintaining inthe aqueous ammonia solution in physical contact therewith a supply of achromate salt of lead soluble in the solution only with difliculty andhaving bound therein a corrosion resisting chromate radical inactive dueto the insolubility of said lead salt, and adding to the solution analkali nitrate eifective to inhibit corrosive action which upondisintegration thereof will liberate said corrosion resisting chromateradical by chemical action.

10. In the art of refrigeration employing an absorption refrigerationsystem formed of ferrous metal and utilizing ammonia as refrigerant andwater as an absorbent, that improvement which comprises maintaining inthe aqueous ammonia solution in physical contact therewith a supply ofchromate salt of lead soluble in the solution only with difficulty andhaving chemically bound therein an inactive chromate radical capable offorming a readily soluble corrosion inhibiting chromate salt uponliberation from the said lead salt, adding to the solution an alkalinitrate which upon dissociation is effective to liberate said inactivechromate radical from the said lead salt to produce said readily solublecorrosion inhibiting chromate salt, and utilizing said readily solublecorrosion inhibiting chromate salt to retard and slow down rate ofdissociation of said alkali nitrate.

11. In the art of refrigeration employing an absorption refrigerationsystem formed of fermucosa:

rolls metal and utilizing ammonia as refrigerant and water as anabsorbent, that improvement which comprises maintaining in the aqueousammonia solution in physical contact therewith a supply of solid leadchromate compound (PbCrO4 or PbCrO Pb),said compound being soluble inthe solution only with difficulty and thereby rendering the chromateinactive as a corrosion inhibiting agent, and liberating the chromatefrom said compound and forming a corrosion inhibiting chromate compoundsoluble in the solution by adding a substance selected from a groupconsisting of an alkali nitrite or an alkali hydroxide;

12. In the art of refrigeration employing an absorption refrigerationsystem formed of ferrous metal and utilizing ammonia as refrigerant andwater as an absorbent, that improvement which comprises maintaining inthe aqueous ammonia solution a solid substance comprising a chromatesalt of lead, and adding an alkali nitrite to the solution.

13. In the art of refrigeration employing an to provide a circuit foractive circulation of aqueous ammonia solution having a corrosioninhibiting chromate salt soluble therein, said parts including a, boileror generator and an absorber comprising a looped coil and vesselconnected thereto, means to provide said soluble corrosion inhibitingchromate salt including a tablet in solid form comprising a substanceselected from a group consisting of strontium chromate (SrCrOn, neutrallead chromate (PbCr04) or basic lead'chromate (PbCrOr, PhD), and meanscomprising a receptacle for holding said tablet in said vessel inintimate contact with the aqueous ammonia solution therein at a regionremoved from the walls defining said vessel.

15. In the art of refrigeration employing an absorption refrigerationsystem formed of ferrous metal and utilizing ammonia as refrigerant andwater as an absorbent, that improvement which comprises providing in thesystem a soluble alkali nitrite and a solid substance selected from agroup consisting of strontium chromate (SrCrOd, neutral lead chromate(PbCrOO or basic lead chromate (PbCrO4, PbO) NILS ERIK WIDELL.

REFERENCES CITED The following references are of record in the iiie ofthis patent:

UNITED STATES PATENTS Number Name Date 1,960,040 Widell May 22, 19342,210,611 Widell Aug. 6, 1940 2,246,665 Buflington June 24, 19412,402,413 Kogel June 1, 1946 2,457,334 Widell --Dec. 28, 1946 FOREIGNPATENTS Number Country Date 117,816 Australia Dec. 1943

1. IN THE ART OF REFRIGERATION EMPLOYING AN HERMETICALLY SEALEDABSORPTION REFRIGERATING SYSTEM FORMED OF FERROUS METAL AND HAVING ANABSORPTION SOLUTION CIRCUIT IN WHICH AQUEOUS AMMONIA SOLUTION ISCIRCULATED THROUGH AND BETWEEN A PLACE AT WHICH AMMONIA IS EXPELLED FROMSOLUTION AND A PLACE AT WHICH AMMONIA IS ABSORBED INTO SOLUTION, THEIMPROVEMENT WHICH COMPRISES MAINTAINING IN THE SOLUTION A SUPPLY OFSOLID LEAD CHROMATE, AND PROVIDING IN THE SOLUTION AN ALKALI NITRITE.